A digital signal is a modified signal based on “1” and “0” data to be transferred and may be a square wave signal for example. FIG. 1 is an example of one bit of a digital signal using a square wave (i.e., a rectangular pulse) signal, as known from the prior art. If a propagation path between sender and receiver circuits is ideal, then the waveform of the square wave signal does not change after propagation and exhibits an ideal waveform shape, shown as a dotted line. The digital signal that is sent represents data of one or more bits depending upon which particular modulation method was used to determine its symbol interval. The receiver demodulates “1” and “0” data by detecting values (i.e., levels) of the square wave signal at every symbol interval T.
An actual transferred digital signal usually has distortions relative to the ideal waveform depending upon characteristics of the propagation path or the rate of the signal. That is, although the information that the digital signal transfers is digital data of “1's” and “0's”, the signal itself is an analog signal. Therefore, a waveform display apparatus (e.g., a test and measurement instrument), such as a digital oscilloscope, real time spectrum analyzer, logic analyzer, or the like, that can sample and store an input signal as digital waveform data is used to display an eye pattern or measure characteristics of jitter exhibited by the input signal.
FIG. 2 is a functional block diagram of an example of a waveform display apparatus, such as a digital oscilloscope, as known from the prior art. A digital signal as a signal under test is provided to a pre-amplifier ATT 10 to adjust the amplitude properly. An analog to digital converter ADC 12 samples the digital signal at an interval that is sufficiently shorter than the symbol interval T to convert the digital signal into digital data in the time domain. A plot of the digital data along a time axis provides a waveform display. The digital data is stored in a memory 14 and then a DSP (digital signal processor) 16 develops the digital data into image data suitable for display of eye patterns described below. The image data are read from memory 14 and displayed as waveforms on a display 18 having a display screen such as a LCD. The DSP 16 also produces spectrum data, necessary for jitter analysis, by FFT calculations performed on the time domain digital data.
A CPU (not shown) controls the waveform display apparatus and a user can enter necessary settings into the waveform display apparatus via an operation panel or mouse. OS (operation software) that is also used in a PC is installed into a large memory storage means, such as an HDD (i.e., hard drive). Many kinds of application software are installed on the OS and opened as software windows to execute various functions. Memory 14 may be realized by RAM or the HDD.
FIG. 3 is an example of displaying an eye pattern of the digital signal using the waveform display apparatus, as known from the prior art. In this case, the digital signal may be a 32-bit signal transferred as packets and the eye pattern is a repetitive overlaid display of waveforms of the bits. In the eye pattern display, larger distortion in the waveforms leads to a displayed eye area that is smaller than normal or the eye diagram may exhibit a larger difference in shape from the ideal shape. Therefore, shape or area of the eye are measured to measure quality of the digital signal. U.S. Pat. No. 6,806,877 discloses such an invention, for example.
Eye pattern presentations typically show frequency of signal occurrences as a histogram using, for example, seven colors of the spectrum. That is, a color which is closer to red means a higher frequency of occurrences of the signal at a given pixel and a color closer to violet means a lower frequency of occurrences of the signal at a given pixel. The eye diagram is also called an eye pattern.
When measuring the quality of a digital signal, the jitter of rising and failing edges is often analyzed and the results displayed as well-know graphs, in addition to displaying an eye pattern. For example, FIG. 4 is a time trend display of jitter, having time as the horizontal axis and jitter magnitude as the vertical axis, as known from the prior art. A time trend display shows how the magnitude of the jitter changes according to time variation. FIG. 5 is a histogram showing jitter frequency, as known from the prior art. The horizontal axis is divided into “bins”, wherein each “bin” represents a deviation in time (either plus or minus) from a center location of zero deviation. The vertical axis indicates the frequency of occurrences (i.e., hits) in any one bin. If the jitter is caused by heat in the device under test, then the histogram draws a Gaussian curve that has the peak around jitter quantity zero. But if the jitter is dependent upon patterns of 0's and 1's it can be a histogram as shown in FIG. 5. In addition, such DSP capability provides statistical data for jitter frequency analysis, such as jitter spectrum and Max/Min numeric values, as graphs or numeric values.
In a conventional eye pattern of a waveform comprising 32 bits, a waveform for each of the 32 bits is drawn (i.e., displayed) such that all 32 waveforms are overlaid. Currently, there is no way to display only those eye pattern or a graph of jitter analysis results that reflect specific bits that may be of interest to a user.
What is desired is a waveform display apparatus and method that can display eye pattern or jitter analysis results relating to specific selected portions of a waveform.